The industrial processes heretofore known for producing 4,4'-dihydroxydiphenyl sulfone include one wherein phenol and sulfuric acid are subjected to a dehydration reaction. This process, however, gives a large amount of 2,4'-dihydroxydiphenyl sulfone as an isomer by-product along with 4,4'-dihydroxydiphenyl sulfone and involves difficulty in isolating 4,4'-dihydroxydiphenyl sulfone from the resulting isomer mixture, so that the products generally obtained invariably contain about 20 wt. % of the by-product. Accordingly, the 4,4'-dihydroxydiphenyl sulfone obtained by the process in no way fulfils the above requirement.
Various processes have therefore been proposed for preparing high-purity 4,4'-dihydroxydiphenyl sulfone. For example, Examined Japanese Patent Publications SHO 38-5274, SHO 43-24660, SHO 47-43936, etc. disclose processes wherein the 4,4'-dihydroxydiphenyl sulfone obtained by the above conventional process and containing a large amount of the isomer is recrystallized with a solvent. Any of these processes nevertheless relates to the separation and purification of the product resulting from the dehydration of phenol and sulfuric acid, accordingly inevitably permits the dehydration to produce about 20% of the isomer, consequently gives the desired 4,4'-dihydroxydiphenyl sulfone in a reduced yield and requires an additional purification procedure and further cumbersome treatment of the separated residue containing a large quantity of 2,4'-dihydroxydiphenyl sulfone.